1. Field of the Invention
This invention relates to the field of disposable thermometry.
2. Description of the Prior Art
In an attempt to make an inexpensive accurate disposable clinical thermometer, those skilled in the art have relied on a metal--generally aluminum or alloys containing aluminum--to transfer energy quickly from the interior of the mouth to an area of the thermometer having various cavities, each containing a corresponding number of heat-sensitive composition of matter which revealed upon changing phase (i.e., melting) a marker beneath, or which were absorbed into another layer in the cavity to indicate that a particular temperature has been reached. While plastics were relatively inexpensive to metals, it was seen that a good heat conductive substrate was necessary to transfer sufficient energy to the cavity within a reasonable time--such as a minute or two--to "fire" or melt the proper number of compositions of matter. See, in particular, U.S. Pat. No. 3,980,581 to Godsey et al. (col. 3, lines 15-36), which recommends aluminum over copper, silver, gold, or stainless steel and the statement that "Naturally, the heat-sensitive carrier 11 must be a material which has a high thermal conductivity and relatively large surface area of contact with the test subject . . . ". Similar recommendations are given at col. 2, lines 36-57 of U.S. Pat. No. 3,704,985 to Pickett et al.; col. 2, lines 60-73 of U.S. Pat. No. 3,665,770; British Pat. No. 1,367,703 (package comprises using a layer of foil); col. 2, lines 38-43; col. 6, lines 24-30, and col. 7, line 55 to col. 8, line 29 of U.S. Pat. No. 4,154,106 to Inoue et al. (not admitted to be available prior art here); col. 3, line 64 et seq. of Sagi et al., U.S. Pat. No. 3,946,612. See also U.S. Pat. No. 3,826,141, U.S. Pat. No. 3,677,088 and U.S. Pat. No. 3,922,917.
Some literature and patents vaguely mention or suggest non-metal alternatives, but give no concrete indication or teaching as to the construction of an operable disposable non-metallic thermometer, e.g., British Pat. Specification No. 551,783 (means for using pressure sensitive adhesive tapes and a rubbery adhesive to form an envelope about a temperature-indicating portion of the device, but further considerations of rigidity, diecutting capability and capability with heat sensitive compositions are not made).
Some suggestion of using plastic to partially or completely surround the heat sensitive solvents has been made in various patents; e.g., British Pat. No. 904,846 (inter alia, p. 2, col. 1, lines 34-50, and p. 2, col. 2, line 68; see also p. 3, col. 1, line 30 and the Figure)--use of a plastic tube; British Pat. No. 3640 to Ramsden (which discloses the use of a base consisting of a piece of paper, celluloid, metal or other suitable material coated with or formed with a hollow or recess, containing a substance which is temperature sensitive). Most notably of these, we believe, are Kluth et al. U.S. Pat. No. 3,465,590, and Pecorella, U.S. Pat. No. 3,929,021.
Pecorella discloses the optional use of plastics, such as polyethylene "or a composite of plastic and metal" 10 (with the caveat of having good heat transfer characteristics), but also teaches the requirement of using vent strips 24 to an open end 18 of the thermometer; we think masking layers 14 and 16 would otherwise cause "blisters" by allowing air to gather before desired use of the thermometer), i.e., Pecorella attempts to try to make a thermometer which will store at any temperature and which will not register a temperature, e.g., "fire", until time of use.
Obviously, elimination of the masking layers 14 and 16 and venting areas 24 would each be a measurable advance in the art; two of present applicants have eliminated the need for masking layers in copending application Ser. No. 946,935, now U.S. Pat. No. 4,232,552 incorporated herein, and the instant invention eliminates the need for vents which are mechanically hard to manufacture and are, therefore, relatively costly.
The only non-metallic and plastic commercial prior art thermometer of any significance is Kluth et al., U.S. Pat. No. 3,465,590, shown in FIGS. 1-3. As shown in FIG. 1, the plastic thermometer is comprised of a temperature indicating portion C attached by neck B to a handle A. The temperature indicating portion C has a well 2 from which off-white wafers 4 protrude from a red-colored well 3 comprising substrate layer 5 and having a boundary with well 2. When a thermometer well is "fired", a number can be seen from the cavity. The red-colored well is better seen in FIG. 2. FIG. 3 shows a cut-away portion of a well with a substrate layer 5 and sealant layer 5a covering wafer 4 which comprises a fatty acid material 6.
Attention of the reader of Kluth et al. is specifically directed from col. 4, line 31 to col. 5, line 56, and also to FIG. 3 and col. 4, lines 64-68, which discloses enclosed pockets (as opposed to Pecorella) of "heat-sealable and workable, moisture-impervious, tough, inert, flexible, transparent plastic, such as unplasticized polyvinylchloride". Unfortunately, Kluth et al.'s rigid non-embossable PVC thermometer--a specimen of which is shown in FIGS. 1-3--must be molded and, therefore, cannot be manufactured by the modern web line operations (see copending application Ser. No. 946,935 and FIG. 10 and col. 6, line 15 to col. 8, line 34 of U.S. Pat. No. 4,154,107) as are known by those skilled in the art. The molded PVC or Kluth et al. thermometer did not, therefore, lend itself to being accurate, as only a few pockets 4, 5, 6 in '590 could be manufactured for a given thermometer and, therefore, only a few readings could be placed on the thermometer--usually at 2.degree. or 3.degree. F. intervals (see col. 7, line 71 et seq. and col. 9, lines 25-45, where Kluth et al. admits only a few pockets are possible).
Currently, disposable clinical thermometers on the market typically use an aluminum substrate layer (sometimes called in the alternative base layer or bottom layer) with embossed pockets covered by a sealant layer compromising a heat-activated or pressure-sensitive adhesive--coated plastic material. Prior to the instant invention, the use of an aluminum substrate was generally felt to be needed to allow the energy of the patient's mouth to reach quickly the thermally sensitive compounds, and the practical use of relatively poor heat conductive substrate lacking metal continuity in disposable thermometers was completely beyond the comprehension of those skilled in the art. Surprisingly, this construction, we have found, restricts the energy flow to the aluminum substrate and then allows the aluminum substrate to distribute and dissipate energy to other parts of the thermometer. The present state-of-the-art construction unfortunately also provides insulating pockets of air to exist in the thermometer matrix area, giving use to uneven heat transfer characteristics.